1. Field of the Invention
The present invention is directed to an arrangement for operating a symmetrical radio-frequency antenna, and particularly an arrangement for operating the radio-frequency antenna of a nuclear magnetic resonance tomography apparatus which is connected to an asymmetrical line, with a sheath wave trap provided between the antenna and the line.
2. Description of the Prior Art
Nuclear magnetic resonance (also known as magnetic resonance imaging) devices are known in the art, which produce a tomogram of an examination subject, such as the human body, using the phenomenon of nuclear magnetic resonance. Such devices include a fundamental field magnet system, which aligns the nuclear spins in the examination subject, and also include gradient coils which generate a spatially different magnetic field, and also include a radio-frequency antenna for exciting the nuclear spins and for receiving the signals emitted by the excited nuclear spins during their return to an equilibrium state. Generally the inductance of such an antenna or coil is interconnected with a variable capacitance to form an LC resonant circuit, and the capacitance is then tuned to the desired frequency. A feeder for the antenna also contains a variable coupling capacitance for coupling the resonator to a radio-frequency generator, which is preferably an oscillator with a following transmission amplifier.
It is known that dipoles, directional radiators and other antennas which are low-resistent at the feed point can be fed with coaxial cables. Transmitters are generally designed so that only low-resistant, asymmetrical cables can be connected thereto. If a symmetrical load is to be fed, a balanced-to-unbalanced transformer (balun) must be interposed, the exact tuning thereof usually being relatively complex and time-consuming, given frequency changes.
Coaxial cables are asymmetrically constructed, and because such cables can, as a practical matter, only exhibit low impedance for low losses, only connection at an antinode is considered for feeding antennas. If, for example, the inner and outer conductor of the asymmetrical coaxial cable are connected to the two balanced-to-ground halves of the radiator, the sum of the forward or return current in the inner and outer conductors of the cable differs from zero, because the two radiator halves do not exhibit completely symmetrical properties relative to the environment, for example relative to ground. As a consequence as of this current difference, a so called sheath wave is formed, the amplitude thereof being particularly high when the cable length is in resonance with the wavelength of the envelope wave. To limit the losses by eliminating such sheath waves, it is known to insert a sheath wave trap between the antenna terminal and the cable, the sheath wave trap essentially consisting of an inductor. The best suppression of sheath waves is obtained with a double inductor, consisting of two halves of a cylindrical coil. The antenna is connected between the two halves. Such an sheath wave trap is described in the article "Coax-Speisung Symmetrischer Antennen," Auerbach, DL-QTC, No. 4, 1961, pages 156-158.